A World to Explore

Archive for August, 2011

This week’s fossils are graptolites (from the Greek for written rocks) I found many years ago in the Lebanon Limestone near the town of Caney Springs south of Nashville, Tennessee. They are of the genus Amplexograptus and probably belong to the species A. perexcavatus (Lapworth, 1876).

Graptolites were colonial organisms consisting of hundreds and sometimes thousands of tiny zooids (individuals) connected together in a flexible proteinaceous skeleton (the rhabdosome). They first appeared in the Late Cambrian (around 510 million years ago) and disappeared forever in the Early Carboniferous (around 350 million years ago). Amplexograptus colonies were probably attached to floats so they could drift through the ancient oceans filtering out organic particles; they would be officially “passively mobile planktonic suspension feeders”. They belong to the Phylum Hemichordata, although there have always been disputes about their actual evolutionary relationships. This matters because graptolites are important index fossils for sorting out the age relationships of Lower and Middle Paleozoic rocks.

Graptolites are usually preserved as thin carbonaceous films on dark shales, making them rather hard to see (as my paleontology students will readily agree). The great 18th Century naturalist Linnaeus even said that they were “pictures resembling fossils rather than true fossils”. Sometimes, though, they are found in lighter-colored rocks like limestones, as above. Goldman et al. (2002) found Amplexograptus in limestones preserved in three dimensions, possibly because the limestones were cemented early around them before they collapsed with decay. They even studied this same species from the Lebanon Limestone. The 3-D preservation allows for a much more detailed analysis of the tiny cups (thecae) which held the individual zooids. It is possible that I could dissolve the limestone shown above and retrieve some delicate three-dimensional graptolites — but I could also just as easily destroy them.

Amplexograptus perexcavatus was originally described in 1876 by the famous geologist Charles Lapworth (1842-1920), who referred it to the genus Diplograptus. Actually, he had two species in his D. perexcavatus group, so it took some taxonomic detective and legal work to fix the current naming system. Lapworth, who I’ve figured below with an inset of his not-very-helpful diagram of the original D. perexcavatus, is well known by paleontologists for his work with graptolites as index fossils. Scientists and historians of science know him as the man who invented the Ordovician Period in 1879 to solve a bitter dispute between Roderick Murchison and Adam Sedgwick who each claimed the same rock interval in Wales for the Silurian and Cambrian periods respectively. Lapworth’s primary biostratigraphic argument for the Ordovician as a separate period was the distribution of graptolites, including our friend Amplexograptus perexcavatus. (Murchison and Sedgwick were long gone by the time their dispute was settled.)

We had a familiar trilobite last week, so this week we’ll look at a poorly-known part of a trilobite: the hypostome. Above is an incomplete forked, conterminant hypostome of the large trilobite Isotelus. (Isotelus, by the way, is the state fossil of Ohio. Do you know your state fossil?)

Hypostome means “under mouth”. On trilobites it is found underneath the cephalon (head) near what we think was the mouth. They are not common in the fossil record. It is obvious from their color and composition that they are part of a trilobite, but most people don’t know about this little plate on the otherwise soft underside (the ventral side) of the animal. The hypostome is important in some new taxonomic schemes for sorting out the trilobites (Fortey, 1990), and they are useful for interpreting a particular trilobite’s feeding habits (Fortey and Owens, 1999).Trilobite hypostome forms from Wikipedia (via Obsidian Soul). The small green plates are the hypostomes seen against the gray cephalon above. A – Natant: Hypostome not attached to doublure; aligned with front edge of glabella (shown in red broken lines). B – Conterminant: Hypostome attached to rostral plate of doublure. Aligned with front edge of glabella. C – Impendent: Hypostome attached to rostral plate but not aligned with glabella.

The hypostome of Isotelus is attached to the anterior edge of the skeleton (thus “conterminant”) and has two distally-directed prongs (making it “forked”). Hegna (2010) has recently suggested this hypostome with its unusual shape and terraced outer structure may have been used for grinding food rather than serrating it. Turns out our hypostome has a unique form among the common trilobites!

NORTHEASTERN MINNESOTA – As part of the Cutting Edge workshop on Teaching Mineralogy, Petrology, and Geochemistry, I had the opportunity to participate in a field trip to the Midcontinent Rift System in northeastern Minnesota. You can imagine how exciting this was to an Ohio-based (outcrop-deprived) petrologist! Here’s a quick tour of some of the spectacular stops on the trip:

Multiple generations of folding in the Soudan-type Banded Iron Formation.

Basaltic pillow in the Ely Greenstone with exquisitely preserved glassy textures in the hyaloclastite rims.

Lichen-covered columnar joints in basalt flows of the North Shore Volcanic Group.

A large plagioclase crystal (showing striations) from an anorthosite xenolith.

Learning about the fantastic geology of northeastern Minnesota was only one of our primary goals. The other was to discuss strategies for field-based teaching and learning. There are countless reasons for bringing our students to the field, including two that I consider most important: (1) the field challenges students to exercise their higher-order critical thinking skills (making observations and dealing with complexity); and (2) the field inspires students to learn and be curious of the natural world. The field trip provided an opportunity for “a collection of North America’s greatest geoscience educators”* to share effective teaching ideas for helping students achieve these goals. I’ve compiled a short list of useful tips that I took away from the trip:

Enhance the social learning environment – Encourage students to learn something about each other by asking them to line up in order of first name or longitude of their place of birth. Establishing a community enhances peer-to-peer learning, which is a powerful instructional practice in the field.

Set and assess goals – For every field trip (and every stop), set specific learning goals, then ask How do we accomplish these goals?How do we know we’ve accomplished these goals? Learning goals and teaching activities will vary with audience and setting. Consider the following example: the learning goal of one particular field trip stop was to observe and describe volcanic features of an outcrop of mafic metavolcanic flows. For the activity, we were provided with an outline of the outcrop and asked to map the distribution of volcanic features from one end to the other. At the end, we compared maps and discussed similarities and differences. The conversation helped us describe the volcanic features we used to define the different units and to determine the stratigraphic “up” direction.

Connect the classroom to the field – We’ve all held large maps against the vans to use as visual aids when explaining the geologic setting, but why not laminate poster-sized photomicrographs or phase diagrams to enhance the discussion? By directly applying field observations to concepts they’ve learned in class, students will learn to transfer content knowledge to different settings.

Don’t neglect the affective domain – An honorable learning goal is to inspire a sense of awe in students. We certainly had several stops that were breathtakingly beautiful and “reminded all of us why we love geology.” I’m sure most of us can recall a particular field experience that motivated us to pursue geology as a major. Sometimes, it’s the “fun” stops that have the greatest impact.

Technology can enhance student learning – One person suggested recording podcasts that students could listen to in the vans between field trip stops. Another person mentioned that smartphones or iPads could be used to take and annotate images of outcrops, which can be loaded onto a wiki later for a collaborative learning project. (The idea probably works equally well for paper photos). On the field trip, we used a portable XRF to measure and compare the compositions of different rock units.

The BIF was one of the stunning outcrops where the field trip leaders allowed us to be excited and explore before trying to discuss the geology.

Geologists and their toys: playing with the pXRF.

If you have ideas for effective field-based teaching practices or would like to share your experiences, please comment!

*Directly quoted response from field trip leader J. Goodge when a passer-by asked “What is this?”

I’ve avoided having a trilobite as Fossil of the Week because it seems like such a cliché. Everyone knows trilobites, and they are the most common “favorite fossil” (invertebrate, anyway). Plus our best trilobite (seen above) is the most familiar trilobite of all: Elrathia kingii (Meek, 1870). One professional collector — just one guy — said that in 20 years he sold 1.5 million of these.

Still, trilobites are cool. They virtually define the Paleozoic Era, appearing in the Early Cambrian and leaving the stage (with so many others) in the latest Permian. They were arthropods, sharing this very large phylum with insects, spiders, crabs and centipedes. The name “trilobite” means “three lobes” referring to the axial lobe (running down the center along the length of the animal) and the two pleural lobes, one on each side. They also have three parts the other way: a head, thorax and pygidium (the tail end).

Elrathia kingii is a ptychopariid trilobite found in extraordinary numbers in Middle Cambrian dark shales and limestones. There is a geological story here, two of them, in fact. One reason they are so common is that their populations were commonly buried by sediment stirred up in massive storms (Brett et al., 2009). They are among the only fossils found in organic-rich dark sediments because they lived in the harsh “exaerobic zone” at the very minimum of oxygen needed for animal life (Gaines and Droser, 2003). They apparently were the first large invertebrates to exploit this marginal environment.Elrathia kingii gives us the opportunity to meet a pioneering American paleontologist: Fielding Bradford Meek (1817-1876). He originally described this species in 1870, calling it Conocoryphe kingii (see above). Paleontologists are quite familiar with the name “Meek” following a fossil species because he described hundreds of them. Meek was a native of Madison, Indiana, a place where Ordovician fossils are abundant and easily collected. He was apparently an unsuccessful businessman so he jumped at a chance in 1848 to work for the U.S. government surveying the geology of Iowa. Meek was good at this job and soon was working with James Hall in New York, the country’s premier paleontologist. Meek was eventually based in Washington, D.C., with the United States geological and geographical surveys. After many accomplishments in government service, he died of tuberculosis in 1876 (White, 1896).

According to the Cutting Edge: "Core of Ely Greenstone outside Pillsbury Hall, home of the Geology & Geophysics Department at the University of Minnesota. Metamorphosed Archean basalt pillows are visible in the core, which is approximately 2 meters tall. Photo by Sharon Kressler." Aka: What I want for my next birthday!

The simple camera utility built into the iPad2 makes recording quick images of specimens in their drawers and trays very easy. The LED backlighting makes the large image on the screen brighter than the actual view, which you can see in the photo above. Combine this with the light weight of the computer and it is actually easier to use the iPad2 for these images than a standard digital camera. The quality of the iPad2 image is not as good as that from a camera (with flash or auxiliary lighting), but we’re just collecting initial views and labels with this process (see below).
As with the images collecting during fieldwork, I can easily annotate the museum photographs with a program like Sketchbook Express. (Circling features for later detailed camera photography is what I do most often — museum staff typically don’t like us drawing on the specimens themselves!) The Boxwave Stylus remains my favorite drawing tool on the iPad2.

The iPad2 is also the most convenient repository of scientific papers I have ever used. It can store thousands of pdfs for quick referencing. The luminous screen makes reading them in the typical dingy light of museum collections easy, and images and text can be expanded for more visual detail. (Try that with paper!) A laptop certainly does these same things, but far more awkwardly while hunched over a museum drawer. The program I use for pdfs is PDFReader Pro (with pdfs transferred through Dropbox), but my friend Bill Ausich at The Ohio State University showed me that iBooks is just as easy to use.

I like to store specimen images from other sources to compare to the fossils in the drawers before me — making identifications this quickly helps me decide which specimens to borrow for later work. I can store the images as jpg files in the iPad2 Photo library or view them as pdfs in PDFReader Pro.

If you can get a wireless connection in the museum (often quirky in old buildings), searching for the meaning of strange stratigraphic terms and archaic species names is obviously of great value. Again, tapping on the iPad2 held in one hand while looking at the specimens themselves is a qualitatively different experience than returning to the laptop in some dark corner. (I could never have guessed that there would be a computer more convenient than my trusty MacBook Pro.)

In summary, the iPad2 has quickly become indispensable for my paleontological work in museums.

WOOSTER, OH – After a month of hard work, the Iceland Keck group parted ways on Saturday. We arrived in Wooster immediately after returning from Iceland and put in a solid week of work in the lab, preparing our samples for thin sections and XRF analyses. In one week, the students produced over 120 thin section billets, powders, and pressed pellets, and almost as many glass beads. Even though the work was tedious and the hours were long, I think we’re all glad that we’ll have data at the start of the school year. Well done, team!

A dessicator full of pressed pellets ready to analyze on the XRF.

Challenges of lab work. We tracked the number of samples that were prepared. This student prepped 20 pressed pellets "of varying degrees of brilliance + some epic failures."

Katharine works her magic on the scale.

Erica grinds the saw marks off of her sample.

Nina presses a brilliant pellet.

Thad oxidizes his samples in the muffle furnace.

Emily celebrates a glass bead that hasn't cracked.

Brennan and Katie troubleshoot the GIS file.

Of course, our work isn’t complete. Once we have the chemistry and thin section observations, we can put the data into the context of the mapped field relationships to understand the volcanic history of one of the oldest central volcanoes in Iceland. We’ll have much to present at the Keck Symposium in the spring and are already looking forward to our reunion.

Tiny little trace fossils this week in a Jurassic crinoid stem from the Matmor Formation of the Negev Desert. They are borings produced by barnacles, which are sedentary crustaceans more typically found in conical shells of their own making. These barnacles are still around today, so we know quite a bit about their biology. (More on how in a minute.) These acrothoracican barnacles drill into shells head-down and then kick their legs up through the opening to filter seawater for food. They’ve been doing it since the Devonian Period (Seilacher, 1969; Lambers and Boekschoten, 1986).

This particular trace fossil is Rogerella elliptica Codez & Saint-Seine, 1958. It is part of a diverse set of borings in the Matmor Formation (Callovian) of Hamakhtesh Hagadol, Israel, recently described in Wilson et al. (2010).

We know so much about boring barnacles because Charles Darwin himself took an almost obsessive interest in them early in his scientific career. While on his famous voyage in the HMS Beagle, Darwin noticed small holes in a conch shell, and he dug out from one of them a curious little animal shown in the diagram below.

Cryptophialus Darwin, 1854

He called it “Mr. Arthrobalanus” in his zoological notes. He figured out early that it was a barnacle, but he was astonished at how different it was from others of its kind. He later gave it a scientific name (Cryptophialus Darwin, 1854) and took on the problem of barnacle systematics and ecology. Eight years and four volumes later his young son would ask one of his friends, “Where does your father do his barnacles?” The diversity of barnacles played a large role in Darwin’s intellectual development and, consequently, his revolutionary ideas about evolution (Deutsch, 2009).

Burrowing barnacle diagram from an 1876 issue of Popular Science Monthly.